1. Field of the Invention
This invention relates to electrochemical devices and components thereof. In one aspect, this invention relates to membranes for use in electrochemical devices. In another aspect, this invention relates to ion exchange membranes for use in electrochemical devices. In still another aspect, this invention relates to a method for producing ion exchange membranes.
2. Description of Related Art
Conventionally, depending upon the type of ionic groups attached to the membrane matrix, ion exchange membranes, also sometimes referred to as mosaic membranes, may be either cation exchange membranes in which cations are transported from one side of the membrane, through the membrane, to the opposite side and anions are rejected for transport through the membrane or anion exchange membranes in which anions are transported from one side of the membrane, through the membrane, to the opposite side and cations are rejected for transport through the membrane. Cation exchange membranes have negatively charged ionic groups, such as —SO3−, —COO−, —PO32−, fixed to the membrane backbone while anion exchange membranes have positively charged ionic groups, such as —NH3+, —NRH2+, —NR3+ fixed to the membrane backbone. Ion exchange membranes may be further categorized based upon the way in which the charged groups are connected to the membrane matrix or based upon their chemical structure. In particular, when the charged groups are chemically bonded to the membrane to the membrane matrix, they are referred to as being homogeneous, and when the charged groups are physically mixed with the membrane matrix, they are referred to as being heterogeneous. The majority of practical ion exchange membranes are homogeneous and composed of either hydrocarbon or fluorocarbon polymer films with which the ionic groups are attached.
In addition to polymeric ion exchange membranes, ion exchange membranes may also be prepared using inorganic materials, such as zeolites, betonite and phosphate salts. However, such membranes are expensive to produce and have several disadvantages including poor electrochemical properties and pores which are too large. Whereas such inorganic membranes are generally undesirable to use due to the aforementioned disadvantages, ion exchange membranes prepared from polymers into which inorganic components have been incorporated have been found to possess chemical stability and high conductivity. Such inorganic-organic (hybrid) ion exchange membranes may be prepared by a variety of methods including sol-gel processes, intercalation, blending, in situ polymerization, and molecular self-assembling.
Bipolar ion exchange membranes consist of at least a layered ion-exchange structure composed of a cation selective layer having negative fixed charges and an anion selective layer having positive fixed charges. These membranes may be used, for example, for separation of mono- and divalent ions, anti-deposition, Anti-fouling, and water dissociation applications.
Amphoteric ion exchange membranes contain both weak acidic (negative charge) groups and weak basic (positive charge) groups randomly distributed within a neutral polymer matrix. The sign of the charge groups in these membranes shows a pH response to an external solution.
In contrast to amphoteric membranes, a charge-mosaic ion exchange membrane has a set of anion exchange elements and cation exchange elements arranged in parallel which provide continuous ion transport pathways between bathing solutions on opposite sides of the membrane. A gradient of electrolyte concentration across the membrane results in parallel flow of anions and cations through their respective ion transport pathways, resulting in a circulation of current between individual ion-exchange elements. Due to the current circulation, charge-mosaic membranes show negative osmosis and salt permeability much greater than their permeability to non-electrolytes.